A monochromator is a device utilized to selectively filter a single frequency from a beam of radiation and to selectively pass only radiation of the selected frequency on for further experimental use. Many conventional monochromators capable of operating in the ultraviolet and x-ray regions of the electromagnetic spectrum utilize a Rowland circle configuration with the incident light striking gratings, used to scatter the incident light, at a grazing angle. The major disadvantage of the Rowland circle configuration is the necessity for the continuous change in position of either the exit or the entrance path of light in the instrument as the instrument is scanned through various frequencies.
Another type of monochromator, capable of use in the x-ray region and having fixed entrance and exit locations, has been developed and is known as the double crystal monochromator. This type of monochromator, the typical optical path of which is similar to that illustrated in FIG. 2 of this patent application, focuses the entering light on a series of two oppositely oriented crystals so that radiation of the selected frequency arrives at the fixed exit location. An example of an illustration of such a monochromator can be found in Nuclear Instruments and Methods, Volume 172 pages 227-236, published in 1980. A similar monochromator was described in The Review of Scientific Instruments, Volume 52 (4), page 509, published in April of 1981. A grazing incidence monochromator based on the double crystal design but using diffraction gratings and capable of operating in the ultraviolet region has also been developed, as exemplified by the design shown in The Review of Scientific Instruments, Volume 43 (3) page 434 published in March of 1972. It is a limitation on each of these designs for monochromators that they are capable of operating only within a limited frequency range, either in the ultraviolet or x-ray regions, and that these designs are not inherently capable of operation in an ultra-high vacuum. Because many wavelengths within this general spectral region are strongly absorbed by air and by window materials capable of supporting partial or full atmospheric pressure, and because some of the most powerful and versatile sources of continuous ultraviolet and x-ray radiation, i.e. synchrotron radiation sources, operate strictly in ultra-high vacuum, it is important for an efficient operation of a monochromator operating in this region that the operation of the instrument be feasible in an ultra-high vacuum, in order to maximize the intensity of the radiation at the target site.
It is another desirable feature of a monochromator of the general type described here that it operate in what is known as the "blaze" condition. The blaze condition refers to the physical situation in which the radiation diffracted from the grating crystal is at its maximum intensity at the desired frequency. The condition for blaze operation using a grating is illustrated in FIG. 1. In FIG. 1, the reference designations N.sub.g and N.sub.f indicate angles which are perpendicular to the grafting surface and to the surface of the grating groove respectively. The angles .alpha. and .beta. are the angles of incidence and diffraction, respectively, of the light hitting the grating. The angles designated by .sigma.and .sigma.' are the angles of equal inclination relative to N.sub.f made by the incident light and the defracted light at the blaze wave length, respectively. In this figure, the grating spacing is indicated by D which is the inverse of the groove density as expressed in grooves per centimeter. FIG. 1A shows a similar incident and reflected beam in the case of Bragg Diffraction for a crystal. This comparison illustrates the essential similarity of the Bragg diffraction condition for a crystal where the path difference for rays reflecting from two successive crystal planes in 2D sin .sigma., and the on blaze diffraction condition for gratings in a monochromator of the type described here, where the path difference between rays reflected from adjacent groove facets is 2D sin .theta..sub.B sin .sigma..
In either of the conditions illustrated in FIGS. 1 and 1A, as long as .sigma. equals .sigma.', the grating or the crystal is operating at blaze condition, which is the condition for maximum efficiency and maximum intensity of the diffracted wavelengths. It is desirable in a monochromator that the gratings or crystal be oriented so that the blaze condition is maintained throughout the entire spectral range of the instrument, by maintaining the condition that the angles .sigma. and .sigma.' are equal. Generally, conventional grating monochromators operate such that the grating is at a blaze condition only at one specific design wavelength, and thus the efficiency of the grating and the overall efficiency of the monochromator are at a maximum value only at this one selected wavelength. One monochromator has been described, in the above referenced article in the Review of Scientific Instruments in March, 1972, which operates continuously at a blaze condition, but only through a limited spectral range and only using a grating for a diffraction element. No grazing monochromator of previous design is known which is capable of operating at blaze over a wide spectral range and in an ultra-high vacuum environment.
Typically, double crystal monochromators for x-ray radiation operate in an environment of helium or some other inert gas. The use of a gas at or near atmospheric pressure in the monochromator simplifies the control of the mechanisms for adjusting the optical elements of the device. However, in order for radiation from a synchrotron to be introduced into such a non-vacuum monochromator, with synchrotrons normally operating in an ultra-high vacuum, the monochromator must be separated from the interior of the synchrotron by a metallic window, typically of beryllium. The x-ray absorption characteristics of beryllium then set a lower limit on the energy of the x-ray photons which can be handled by such a double crystal monochromator. By contrast, the efficiencies of gratings, and the extreme grazing angles at which they must be used usually, set an upper limit on the photon energies which can be used with a grating monochromator. The junction of these limitations, generally in the range of 1 to 2 keV, is an important spectral region not conveniently accessible using either of these types of equipment. No prior ultra-high vacuum monochromator is known to have been constructed which is capable of operating in both the x-ray and ultraviolet regions and capable of utilizing either crystals or gratings, or both, at alternative time periods, as the optical elements in its optical system.